There has been a demand for the reduction in volatile organic compounds (VOC) released from coating compositions from the viewpoint of global environmental protection, and there have been ongoing rapid shifts from solvent-based coating compositions to aqueous coating compositions in various fields.
A considerable amount of solvent-based coating compositions were previously used also in automobile coating, and reduction of VOCs released from those coating compositions was an urgent matter. A range of coating compositions used in automobile coating steps for undercoating, intermediate coating, and top coating have shifted from organic-solvent-based coating compositions to aqueous coating compositions, and coating with aqueous coating compositions has become predominant.
Additionally, in order to decrease the environmental load, there has been an approach to shortening the coating step by omitting part of the heat-curing step (mainly omitting the heat-curing step performed after intermediate coating), which used to be performed for each of the intermediate coating film layer and the top coating film layer. Automobile coating performed in such a shortened step is now becoming mainstream.
Typically, an automobile body, in particular its exterior panel part, is provided with a multilayer coating film composed of an undercoating film excellent in corrosion resistance, an intermediate coating film excellent in smoothness and chipping resistance, and a top coating film excellent in appearance and environmental load resistance, for the purpose of imparting excellent corrosion resistance and appearance to the body.
Of the coating compositions, intermediate automobile coating compositions, in particular, are required to exhibit excellent chipping resistance (chipping: coating film damage caused by small pebbles on the road being kicked up and hitting the coating film), adhesion to the undercoating film, storage stability (pigment dispersibility and circulation stability in the coating line pipe), finished appearance, coating workability (popping resistance and sag resistance), etc. The market demands that all of these requirements be met at high levels, with the recent shift into aqueous compositions and even the shortened coating step being addressed.
For example, PTL 1 discloses an aqueous resin composition containing (A) a polyester resin having an acid value of 10 to 50 and a hydroxy value of 20 to 150, (B) a vinyl-modified polyester resin having an acid value of 20 to 100 and a hydroxy value of 20 to 150, and (C) a curing agent, wherein the polyester resin (A) includes a structural unit derived from an aromatic acid and an alicyclic acid; and the vinyl-modified polyester resin (B) contains a fatty acid chain to which a vinyl polymer part is bound and is composed of a specific amount of the vinyl polymer part and a specific amount of a structural unit derived from an α, β-ethylenically unsaturated monomer containing carboxyl groups.
However, a multilayer coating film containing an intermediate coating film formed of the aqueous coating composition of PTL 1 may result in insufficient chipping resistance or insufficient levels of coating workability (popping resistance) due to the lack of flexibility, for example.
PTL 2 discloses an aqueous curable resin composition containing a (meth)acrylic-modified polyester resin, (meth)acrylic resin particles having an average particle size of 0.1 to 2 μm, a melamine resin, and water.
However, a multilayer coating film containing an intermediate coating film formed of the aqueous curable resin composition of PTL 2 may result in insufficient chipping resistance or insufficient levels of finished appearance due to the influence of (meth)acrylic resin particles.
PTL 3 discloses an aqueous intermediate coating composition containing a resin component and a curing agent, wherein the resin component is a graft polymer having a polyester resin as the main polymer and an acrylic resin as a branch polymer.
However, a multilayer coating film containing an intermediate coating film formed of the aqueous intermediate coating composition of PTL 3 may result in insufficient levels of finished appearance and coating workability (popping resistance), and poor circulation stability in the coating line pipe.